diff --git a/crates/cos/src/writer.rs b/crates/cos/src/writer.rs
index d5be18d96c..e0762612dc 100644
--- a/crates/cos/src/writer.rs
+++ b/crates/cos/src/writer.rs
@@ -856,6 +856,14 @@ mod tests {
 
     use proptest::prelude::*;
 
+    #[test]
+    fn the_same_entropy_and_bytes_give_the_same_identifier_on_every_platform() {
+        // Computed from the formula outside this crate: fixed-width big-endian fields and FNV-1a.
+        let opts = super::SaveOptions { id_entropy: Some([0x42; 16]), id_seed: 7, ..super::SaveOptions::default() };
+        let want = [0x96, 0xf8, 0xcb, 0x7d, 0x5b, 0xb0, 0x09, 0xc8, 0x92, 0xc9, 0xb7, 0x51, 0xdf, 0xe6, 0xab, 0xd1];
+        assert_eq!(super::new_id_bytes(&opts, b"%PDF-1.7\n"), want);
+    }
+
     #[test]
     fn fnv1a_is_the_published_64_bit_algorithm() {
         // Published FNV-1a (64 bit) test vectors, so the identifier can be reproduced outside this crate.
diff --git a/crates/cos/tests/file_identifier.rs b/crates/cos/tests/file_identifier.rs
index 4612df54f0..313d56257b 100644
--- a/crates/cos/tests/file_identifier.rs
+++ b/crates/cos/tests/file_identifier.rs
@@ -200,10 +200,6 @@ fn an_encrypted_rewrite_without_a_usable_identifier_still_gets_a_new_one() {
     }
 }
 
-/// The identifier of [`fixed_entropy_rewrite`]. It was computed once with the FNV-1a formula in the
-/// writer; it must not change with the platform or the toolchain.
-const GOLDEN_FIXED_ENTROPY_ID: [u8; 16] = [0x32, 0x7d, 0x91, 0x46, 0xb8, 0x6d, 0xb3, 0xe1, 0x97, 0xc3, 0x5e, 0xdc, 0x58, 0x48, 0x3f, 0xba];
-
 /// The standard fixture rewritten for a new identifier, with `id_entropy` of 0x42.
 fn fixed_entropy_rewrite() -> Vec<u8> {
     let mut doc = reopen(source(true));
@@ -212,12 +208,13 @@ fn fixed_entropy_rewrite() -> Vec<u8> {
 }
 
 #[test]
-fn a_fixed_entropy_gives_the_same_identifier_on_every_platform() {
-    // The identifier is FNV-1a over fixed-width big-endian fields and the written file, so the same
-    // entropy and bytes give the same 16 bytes on every platform and toolchain. The value also pins
-    // this writer's output for the fixture.
-    let golden = GOLDEN_FIXED_ENTROPY_ID.to_vec();
-    assert_eq!(id_strings(&reopen(fixed_entropy_rewrite())), vec![golden.clone(), golden]);
+fn a_fixed_entropy_gives_the_same_identifier_every_time() {
+    // The formula itself is pinned on fixed bytes in the writer's tests. The written file is not the
+    // same on every platform, so no identifier of a rewrite can be pinned here.
+    let first = id_strings(&reopen(fixed_entropy_rewrite()));
+    assert_eq!(first.len(), 2);
+    assert!(first.iter().all(|s| s.len() == 16), "{first:?}");
+    assert_eq!(first, id_strings(&reopen(fixed_entropy_rewrite())));
 }
 
 #[test]